The present invention relates generally to a microporous membrane, methods of making the microporous membrane and, in particular, to the use of the membrane in making batteries. For the purpose of the present invention, a microporous membrane and a separator refer to the same structural elements of a battery.
Electrically-powered automotive vehicles, such as automobiles, buses and trucks, are more environmentally friendly since they do not discharge exhaust gases that contribute to air pollution. These vehicles are conventionally powered by a storage battery pack, which supplies the electrical energy for operating the vehicle on the open road, including charging circuitry to enable recharging of the batteries such as by connection to a conventional electrical supply. However, such vehicles are seriously limited in the distance they can travel between battery charges. The lack of batteries having high energy density and long battery life is one of the major factors hindering a more widespread use of electric vehicles. Moreover, rapid growth in the wireless communication market, along with the need for increased mobility and higher power requirement, also require the development of improved battery technologies.
The lithium-ion battery has been the preferred power sources for various applications because of its higher energy density, longer cycle life of charging-discharging and the absence of a xe2x80x9cmemoryxe2x80x9d effect problem. In the early 1990""s, the first liquid lithium-ion battery xe2x80x9cLLBxe2x80x9d was commercialized by SONY Corporation and the worldwide market of LLB has grown significantly in the last ten years. In 1997, the LLB emerged as the leader in the portable electronics market capturing a significant market share.
The LLB is produced mainly in a spiral wound configuration in which a separator is sandwiched between positive and negative electrode ribbons. The separator used for LLB is a hydrophobic polyolefin based porous polymer such as polyethylene xe2x80x9cPExe2x80x9d, polypropylene xe2x80x9cPPxe2x80x9d, and a trilayer PP/PE/PP (U.S. Pat. Nos. 4,620,956; 5,667,911; 5,691,077). The trilayer PP/PE/PP separator developed and produced by Celgard LLC has been commonly used in LLB production for several years.
A polymer lithium-ion battery xe2x80x9cPLBxe2x80x9d has also been developed for use in portable devices by replacing the liquid electrolyte with a solid polymer or gel polymer electrolyte. Gozdz et al. U.S. Pat. No. 5,418,091, May 23, 1995 and U.S. Pat. No. 5,607,485, Mar. 4, 1997 disclose a plastic battery cell that is made by laminating together a separator between positive and negative electrodes. The separator contains a polymer and a plasticizer, and is substantially devoid of pores. In the process of battery assembly, after lamination at high temperature and pressure, porosity is formed in the separator as well as in the electrodes as a result of extracting the plasticizer with a volatile solvent. As this process requires extraction of the plasticizer, it increases the potential for environmental pollution as well as the cost of making the battery. Moreover, the current collector material for electrodes, such as aluminum foil and copper foil, which have been commonly used for LLB batteries cannot be used for such PLB batteries. The current collect materials of electrodes for the PLB batteries must be in the form of grid or screen of metals such as aluminum grid and copper grid, which increases the cost of production.
Sun, U.S. Pat. No. 5,603,982, Feb. 18,1997 and U.S. Pat. No. 5,69,974, Mar. 11, 1997 disclosed the use of solid polymer electrolyte films which are produced by in-situ polymerization of three monomers, together with a lithium salt and plasticizers in the batteries. The resulting gel polymer electrolyte film is able to adhere to the electrodes of the batteries when applying a vacuum to seal the battery package.
However, as the lithium salts used in Sun is sensitive to moisture, the battery assembly operation has to be performed under anhydrous conditions, for example, in dry box under a nitrogen or an argon or in dry room. This substantially increases the cost of producing these types of batteries.
An improved gel polymer battery has been developed to reduce the cost of production as this type of battery uses the same electrodes as the LLB product, i.e., positive and negative electrode materials coated onto aluminum foil and copper foil respectively, and does not require dry box for battery assembly. A gel polymer lithium-ion battery has been made (Sanyo Corporation) by using same LLB battery electrodes, separator and liquid electrolyte with the further addition of monomers to the liquid electrolyte and subsequently polymerizing the monomers inside the battery case. Although the resulting gel polymer can adhere to the electrodes, the level of adhesion is low and susceptible to separation from the electrodes, and the binding between separator and electrodes could be deteriorated easily during battery operation.
Pendalwar et al., U.S. Pat. No. 5,716,421, Feb. 10, 1998 discloses a gel polymer lithium-ion battery using standard LLB battery electrodes and electrolyte, but replacing the ordinary polyolefin type porous separator such as Celgard(copyright) separator (a polyolefin-based microporous membrane) with a multi-layer coated separator which is produced by coating a polymer layer onto the polyolefin separator. However, this coating process reduces the porosity of the separator as the polymer penetrates into and clogs the pores of the porous polyolefin separator. This, in turn, reduces the charging-discharging rate capability of the battery. Moreover, the binding of the gel to the electrodes is weak and the battery can easily be deteriorated, especially for lager sized batteries such as those for use in automotive vehicles.
One aspect of the invention is directed to a microporous membrane comprising (a) a hot-melt adhesive, (b) an engineering plastics, (c) optionally a tackifier and (d) optionally a filler.
In another aspect of the invention, a microporous membrane is made by (a) dissolving hot-melt adhesive, engineering plastics, and optionally a tackifier in an organic solvent, and then adding a pore former and optionally a filler to form a homogeneous slurry, (b) casting the slurry as a film onto a support substrate, (c) evaporating the solvent from the membrane, and (d) washing the resulting microporous membrane with water to form a microporous membrane. A preferred pore former is lithium bromide. The resulting microporous membrane is particularly useful in the construction of a battery, particularly a lithium-ion battery.
The contents of the patents and publications cited herein and the contents of documents cited in these patents and publications are hereby incorporated herein by reference to the extent permitted.